The present invention relates generally to semiconductor manufacture methods, and more particularly to a method that provides high-quality and quick modification to a substrate to be processed, by utilizing microwave surface-wave plasma. The present invention is suitable, for example, for a method for forming a silicon oxynitride film.
Along with recent fine processing of semiconductor devices, a silicon oxynitride film has been applied in a gate insulating film with a thickness below 3 nm. A silicon oxynitride film is made by introducing nitrogen into a silicon oxide film. The silicon oxynitride film has attracted attention due to its excellent characteristics such as high specific permittivity, leak current prevention, and boron diffusion prevention from a gate electrode.
While the method for manufacturing a silicon oxynitride film includes a method for forming a film of a silicon thermal oxide film first and then introducing nitrogen, and a method for forming a silicon oxynitride film directly on a silicon substrate using a CVD (chemical vapor deposition) method, the former is promising viewed from electric characteristics of an interface with a silicon substrate. In addition, a heat treatment, a plasma process, etc. have been studied as a nitriding process to the silicon oxide film.
One proposed method for making a silicon oxynitride film which utilizes a heat treatment, for example, heats up a wafer under nitrogen monoxide gas atmosphere for several hours (62nd Japan Society of Applied Physics, Annual Meeting, Preprint, No. 2, page 630) so as to thermally nitride a silicon oxide film. Thermal nitriding needs such high temperature as 800° C. to 1000° C., so that nitrogen easily moves in a silicon oxide film and reaches an interface between the silicon oxide film and silicon. Since diffusion differs between the silicon oxidation film and silicon, nitrogen accumulates in the interface of the silicon oxide film and silicon. Thus, a nitrogen concentration distribution in a depth direction in the silicon oxide film as a result of thermal nitriding does not locate nitrogen on a surface, and causes increased nitrogen concentration in the interface between silicon and the silicon oxide film. Another example for making a silicon oxynitride film which utilizes a heat treatment is a nitriding method using NH3, as disclosed in Japanese Patent Publication No. 6-140392.
One proposed method for making a silicon oxynitride film which utilizes the plasma process use remote plasma to sufficiently decrease nitrogen ions in nitrogen plasma, transports only nitrogen active species, and nitrides a silicon oxide film (62nd Japan Society of Applied physics, Annual Meeting, Preprint, No. 2, page 631). This method uses reactive neutral active species to nitride a silicon oxide film at comparatively low temperature of about 400° C. It decreases nitrogen ions in nitrogen plasma and uses only nitrogen active species by maintaining a reactor at a high pressure, and spacing plasma generation part from a wafer. The remote plasma process exhibits a large nitrogen concentration distribution in a depth direction in a silicon oxide film near the surface, and small one at the interface between silicon and a silicon oxide film.
A nitriding method that uses ions from plasma has also be known, as disclosed in Japanese Patent Publication No. 10-173187. This method injects ions with energy below 50 eV, and may obtain a nitrogen concentration distribution in a depth direction that has a peak at a depth dependent upon the ion energy.
These conventional methods for nitriding a silicon oxide film have several disadvantages and have not yet been reduced to practice.
For example, the heat nitriding process has a high nitrogen concentration at the interface of a silicon oxide film and silicon, and causes inferior device characteristics, such as low mobility of a channel in a transistor.
On the other hand, the remote plasma process cannot obtain sufficient nitrogen active species since necessary nitrogen active species decreased with the nitrogen ions in the plasma, and takes a very long process time. In addition, this process cannot enhance the nitrogen surface density since the nitrogen concentration distribution in a depth direction in the silicon oxide film decreases drastically by depth. An introduction of a sufficient amount of nitrogen would need a high temperature, disperse nitrogen deeply, and cannot form a shallow nitrided layer.
The nitriding process using ions at low pressure and temperature leaves damages in a silicon thermal oxide film, deteriorates performance in leak current and boron dispersion prevention. Damages recover at high temperature, but nitrogen disperses in a high temperature process and a shallow nitride layer cannot be obtained. In addition, when ions are implanted with high energy of several tens of eV, ions are injected down to a deep position and cannot nitride an extremely thin oxide film below 3 nm.